Whispering Gallery Modes (WGM) in microspheres are optical resonances taking place in a microcavity having a cylindrical or spherical symmetry. These modes circulate at the edge of a microsphere by total internal reflection where they are affected by external perturbations via the evanescent field energy coupling.
Dielectric microspheres act as resonators with a geometry that offers the highest quality factor (Q up to 109). Light trapped in these resonators by means of total internal reflection exhibits WGMs at specific wavelengths known from Mie theory. In this formalism, these resonances are determined by the radius a of the microsphere and the refractive index contrast m=m1/m0 where m1 and m0 are the refractive indices of the microsphere and the surrounding medium, respectively.
WGM detection requires no microbial culture. It has shown great sensitivity in monomolecular detection. The use of multiple microspheres having different targeted analytes, i.e. bacteria or viruses, allows for detection of multiple infectious diseases with a single analysis.
However, because even slight variations of the radius of the microspheres affect the WGMs and because it is not yet possible to manufacture microspheres with such tight precision, WGM refractometry requires pre-calibration of the WGM spectral displacements for each microsphere against a reference solution. It was therefore mandatory that measurements be carried out on the same microsphere before and after contact with a test solution including the analytes. The microsphere should therefore be fixed on a substrate throughout the test for the position of each microsphere to be retraceable. Fixing microspheres on a substrate however decreases sensitivity.
There is therefore a need for a technique allowing the WGM microbial detection with multiple microspheres targeting various analytes that may flow and move freely in the test solution.